This invention relates to the field of asynchronous data communications, and more particularly, this invention relates to the field of decoding asynchronous digital data that is sent over a wire line, radio, or fiber optic communication channel.
Asynchronous communication systems are commonly used where data is not time oriented and is sent in bursts and received often in a burst receiver. In order to decode incoming data correctly, knowledge of the data amplitude should be ascertained in order to establish a reference within a communications receiver. Any receiver also should typically be capable of performing DC signal restoration during potentially long xe2x80x9cquietxe2x80x9d periods. An example of a xe2x80x9cquietxe2x80x9d period is shown in FIG. 1, where a frame of data has a stop bit, intermediate data (represented by the letter D) and a stop bit, followed by a two minute signal delay, followed by another frame of data with a start and stop bit and intermediate data (D).
Asynchronous digital data is often sent over a wire line, radio link, or fiber optic communication channel. In some prior art systems, analog implementations have been used for decoding the asynchronous signals. Typically, decoding circuits have been designed to take into consideration the length of a communication line or the link loss. In this type of system, the circuit will restore an analog signal to obtain a digital signal value and then make a decoding decision on the digital signal value. Typically, the circuit is designed based on knowledge of the difference in data amplitude coming into a communications receiver, such as in a fiber optic communications system.
In some prior art applications where the link distance is still known with certainty, the receiver could move even if the transmission distance and theoretical amount of attenuation is known. However, the link loss would not be known and the signal strength could be variable. In some prior art asynchronous communication systems, designers typically would know the distance between a transmitter and receiver. For example, the communication line could be from room-to-room, or from city-to-city. Once this link distance is known, then the circuit is designed based on the theoretical amount of signal attenuation. However, there are often times when the amount of signal attenuation cannot be known in a communication line. Also DC signal restoration is difficult.
One of the key aspects of asynchronous data communication is the transmittal of packets or frames. Typically, a burst receiver may be used as mentioned before. In many prior art burst receivers, the receiver will lose its DC reference over a time pause in signal communication, which is why many communication systems use Manchester encoding or pseudo random number generation.
An example of one problem with DC signal restoration is comparing the difference between transmitting asynchronous data over three feet of cable between two computers in the same room, as compared to a longer distance system. In the asynchronous data communication system, if logical 1 corresponds to five volts and logical 0 corresponds to zero volts, there would be little problem in finding the midpoint reference or threshold when the data signal is transmitted through only three to five feet of cable. However, it the data signal is transmitted through 100 miles of cable, then the final attenuation would be great and the final signal could be about 100 millivolts. If the ideal threshold was set when only the three foot cable were used (i.e., a threshold of 2.5 volts), that threshold would be insufficient for the 100 mile transmission length and the threshold value would have to be lowered to about 50 millivolts. Thus, it is evident why the threshold in the most basic prior art asynchronous data communication system was set based on the transmission distance.
Some analog devices that are used for measuring the signal attenuation and setting a signal threshold in a receiver have been implemented with asynchronous data communications, but these systems do not accomplish high dynamic range burst mode asynchronous data decisions because of thermal drifts and parasitic effects of passive and active components.
Other common prior art methods have also been used to establish a signal reference in asynchronous data communication systems.
In a preamble system, several disposable data bits are sent prior to a data payload having the start bit to allow a decision circuit in the communications receiver to establish a signal reference, Thus, as soon as a start bit is transmitted, the circuitry has measured both zero and one logic levels and has set a threshold half-way between the zero and one logic levels.
In a second method known as xe2x80x9cavoidance of risk mode of operation,xe2x80x9d a continuous stream of data is sent with an even mix of ones and zeros by using a randomizing circuit. To avoid a long quiet period, which sometimes occurs such as a two minute delay as shown in FIG. 1, the data is multiplied by a pseudo random number in the transmitter to ensure the data stream is rich in data transitions. Thus, there will always be ones and zeros that are transmitted. Then the communications receiver circuitry decodes the pseudo random number and extracts the information.
In another method known as automatic gain control (AGC), a constant automatic gain control circuit controls the communications receiver gain. For example, if the transmission line is 100 miles long and there is a 100 millivolt threshold, then the signal will be amplified back to five volts. As a result, the system would use a 2.5 volt reference. However, there are time constants. If the system is quiet for extended periods, for example, about two minutes or some other time period, then the signal will decay and there is no signal information on which to make a gain or adjustment of the signal. Thus, the DC signal information is not present.
It is evident that the prior art methods are not adequate in some circumstances. For example, the preamble system reduces the data rate of the channel. As a result, in asynchronous data transmission, the preamble method prevents binary data transfer. Certain characters could be sent as a preamble and would, therefore, not be used for the information content. For example, if bits are sent to a communications receiver in order to synchronize that receiver, then no data information is sent. Thus, the preamble system requires a preamble and prevents binary data transfer. If binary data is transmitted, there could be no start bit. The preamble system would not know the difference between a binary data bit or preamble bit. There could also never be a xe2x80x9csyncxe2x80x9d word in the binary transmission because it would mistake it for a frame signal. In the xe2x80x9cavoidance of the first mode of operation,xe2x80x9d as noted before, a continuous transmission is required for randomized data. The automatic gain control system (AGC) solves the problem of amplitude fluctuations, but does not solve the problem of DC restoration. In an automatic gain control system, it is possible to correct the gain, but it cannot affect the DC reference point. Other prior art digital signal decoding and digital modulated signal reading devices are shown and disclosed in U.S. Pat. No. 5,052,021 to Goto et al., U.S. Pat. No. 4,823,360 to Tremblay et al., and U.S. Pat. No. 4,540,897 to Takaoka et al.
It is therefore an object of the present invention to provide an apparatus and method for decoding transmitted asynchronous data where the distance and loss characteristics of a communications channel are unknown and widely variable.
It is also an object of the present invention to recover digital information of an asynchronous digital data communication system in a wire line, fiber optic, or radio receiver with high dynamic range variations in the length loss.
The present invention is advantageous because it now provides long term circuit stability and accomplishes high dynamic range burst mode asynchronous data decision decoding of asynchronous data communication signals with relatively long periods of absent data without losing a reference. It is possible to use the decoding apparatus and method of the present invention in any product that sends asynchronous digital data over a wire line, radio, or fiber optic communication channel where the distance and loss characteristics of the channel are unknown or widely variable.
In accordance with the present invention, asynchronous data signals are digitally decoded by measuring the amplitude of a signal bit having negative and positive peak values, and digitally computing a mid-bit reference from the signal bit. Changes in the base band signal peak excursions are tracked and the mid-bit reference is updated from the tracked changes of the base band signal excursions. To track thermal drifts and components aging, the positive and negative peak registers are periodically decremented and incremented, respectfully, at a rate several orders of magnitude slower than the ADC sample rate.
In one detailed aspect of the present invention, the apparatus digitally decodes asynchronous communication signals and includes an analog-to-digital converter for converting analog communication signals having negative and positive signal peaks into binary signal values corresponding to the analog communication signals. A negative peak register and a positive peak register store negative and positive peak signal values and the minimum negative and maximum positive signal values corresponding to the minimum, most negative peak signal value and the maximum, most positive signal value. A negative peak comparator and a positive peak comparator compare the positive and negative signal peaks of a currently received binary signal value with the minimum negative and maximum positive peak signal value stored within the negative and positive peak registers so as to update the negative and positive peak registers with any new minimum negative and maximum positive peak signal values.
A subtraction circuit subtracts the positive peak signal value from the negative peak signal value to determine the magnitude signal value corresponding to the difference between a binary zero and one of the binary signal values. An addition circuit adds one-half of the magnitude signal value to the minimum negative peak signal value stored in the negative peak register to obtain a threshold signal value used to determine the mid-bit reference.
In still another aspect of the present invention, the analog-to-digital converter converts an analog communication signal into an N-bit value. The N-bit value can comprise a 12-bit value. A threshold register also stores the threshold signal value. A threshold comparator compares the threshold signal value to a currently received binary signal value corresponding to a currently received and converted analog communication signal. A circuit latch can also be included for latching a single bit from an N-bit value. An analog-to-digital buffer register can store the binary signal values that have been converted from the analog communication signals.
In a detailed method aspect of the present invention, the method of the present invention digitally decodes asynchronous communication signals, and comprises the steps of converting analog communication signals having negative and positive peak values into binary digital signal values. The method also comprises the step of storing in respective negative and positive peak registers at least the negative and positive peak signal values. The method further comprises the step of comparing the negative and positive peak signal values of a currently received binary signal value with the minimum negative and maximum positive peak signal values stored within the negative and positive peak registers, and updating the negative and positive peak registers with the new minimum negative and maximum positive peak signal values when the comparison step determines that there is a new minimum negative or maximum positive peak signal value.
The magnitude signal value corresponding to the peak-to-peak signal excursion is also determined in a subtraction circuit by determining the difference between a digital one and digital zero. The minimum negative peak signal value stored in the negative peak register is added to one-half of the magnitude signal value, resulting in a threshold signal value.
The method further comprises the step of initially measuring the amplitude of the signal start bit. The peak-to-peak excursion is calculated and half of that calculated peak-to-peak excursion value is added to the minimum measured value corresponding to the minimum negative peak signal value to obtain a mid-bit reference.
In still another method aspect of the present invention, the method includes the step of tracking changes of base band signal peak excursions by measuring the currently received or xe2x80x9csampledxe2x80x9d negative and positive peak signal values and comparing those negative and positive peak signal values with previously determined minimum negative and maximum positive peak signal values that are stored within respective negative and positive peak registers. The analog communication signals are converted into a binary signal values with the analog-to-digital converter and stored in an analog-to-digital buffer register. The analog communication sample can be oversampled by about ten times.
The method also comprises the step of converting the analog communication signal into an N-bit value that typically is a 12-bit value. The method also includes the steps of decrementing the positive peak register and incrementing the negative peak register periodically, which in one aspect of the present invention, is about one Hz rate. The method also includes the steps of determining the magnitude signal value corresponding to the difference between a binary zero and one and the binary digital signal by subtracting the positive peak signal value from the negative peak signal value and dividing the magnitude value by two within the division circuit and then adding the result of the division step to a minimum negative peak signal value that is stored within the negative peak register.